High Average Power Laser Program Workshop

1
Pulsed E-beam Thermofatigue System
Chad E. Duty Materials Science Technology
Division Oak Ridge National Laboratory
Helium Retention in Nano-Porous Tungsten
Nalin Parik, R. Parker, J. Gladden University of
North Carolina, Chapel Hill R. Downing, L.
Cao National Institute of Standards and
Technology Gaithersburg, MD
High Average Power Laser Program
Workshop University of Wisconsin, Madison October
22-23, 2008
With a special appearance by the ghost of Jeff
Latkowski
2
Pulsed E-beam Thermofatigue System (PETS)

• Peak Voltage 70 kV (variable)
• Peak Current 74 Amps
• Pulse Width 0.5 to 1.5 µsec (variable)
• Pulse Rise/Fall Time 800 ns
• Pulse Frequency Single shot to 100 Hz
• Duration gt 10 million shots
• Usable Beam Waist 0.565 cm (1 cm2 area)
• Current Density Variation (at UBW) 31

High Voltage Shield
Electron Gun Chamber
Pulser Unit
Calculations courtesy of Ion J.
Blanchard Electron F. Hegeler
Vacuum System Controls
3
FY-08 Goals
Design, fabricate, install heated sample
stage. Demonstrate high-cycle, high vacuum
operation on heated tungsten sample. Prepare
for use with radioactive samples (FY-10 and
beyond.)
4
Sample Holder / Cooling Design

• Functions
• -- Hold sample securely
• -- Permit optical access
• (from both top sides)
• -- Versatile clamping design
• (allow for various sample dims)
• -- Cool sample
• (provide heat sink for e-beam energy)
• -- Heat sample
• (low temp thermal processing / aging)
• -- Electrically ground sample
• (prevents sample from charging)
• -- Measure temperature
• (either fast TC or melt blocks)

5
Sample Holder / Cooling Design
Ceramic Insulator for 8 Conflat (prevents e-
from returning to copper anode )
Sample Holder (previous slide)
Thermocouple / Electrical Feedthrough
Resistive Heater Thermocouple Leads
Path to ground for electrons
Rated 1250W at 1.5 gpm
8
E-beam chamber supplied by HeatWave Inc.
Use oscilloscope to measure voltage drop across a
1 W resistor (1 V 1 A through e-beam)
6
Finite Element Thermal Analysis
Developed preliminary finite element model of
sample holder in ABAQUS.
E-beam Heating Options
Example Case

• Radial Symmetry
• Steady State
• Resistive Heater (250 W)
• Coolant Flow
• Tinf 10-20dC
• hoff 50 W/mK
• hon 5,000 W/mK
• Radiation
• Tinf 20dC
• e 0.2 (free surfaces)
• Conduction
• W 174 W/mK
• Mo 138 W/mK
• Steel 26 W/mK
• Insul 1 W/mK

Equal Inside 738 W/cm2 Outside 738 W/cm2
21 Split Inside 428 W/cm2 Outside 856 W/cm2
E-beam 21 Split Heater 250 W Coolant 5,000
W/mK
7
Steady State Heat Transfer
Heater Off (0 W)
Heater On (250 W)
E-beam Equal (738738)
E-beam Split (428856)

• Due to radial symmetry, natural tendency is for
  center of sample to get hotter.
• Splitting beam produced more uniform surface
  temperature than using the resistive heater.
• Predict 2.61 beam distribution will not be
  problematic (steady state).

8
Components for Holder are Bench Assembled
9
(lack of) Progress since last HAPL Meeting

• Prior to previous HAPL meeting a series of
  vacuum leaks were plugged and issues with power
  conditioning resolved.
• Held 10-8 Torr Vacuum for 5 months
• After resolving issues with radiological
  operation (75 mR/hr operation,) we moved onto
  long duration test of ebeam.
• ----gt immediately and still frustrated by
  operational issues.

10
Interference / Pressure Spike?

• Baseline pressure 1x10-7 Torr
• Pulse at 1 Hz, 30 kV
• Pressure pulse to 4x10-7 Torr
• Pulse in sync with 1 Hz e-beam
• Occasional spike of 7x10-7 Torr (trip)
• Shield cables ground system
• Verify system pressure pulse on separate gauge
• Conclusion Pressure pulse is real
• If so, should bake out overnight

11
E-beam Bake Out

• Pulse overnight at 15 power (12 kV)
• Pressure pulse decreased by 0.6x10-7 Torr
• Increased trip point from 40 to 70 power
• Pulse overnight at 50 power (38 kV)
• Pressure pulse decreased by 0.3x10-7 Torr
• Increased trip point from 70 to 100 power

Radiation Measurement 100 power (74 kV) 1 Hz
pulse rate 1 µs pulse width 75 mR/h at contact
12
Chamber Leaks Water Vapor

• Condition the chamber by running e-beam
• After 4 days of running, pressure unstable
• Beam expanding to hit chamber walls
• Excessive heat on end plates / gaskets
• Surfaces oxidizing thermally cycling
• Water vapor and leaks due to thermal expansion
  need to be dealt with.

No sample
Leaks
13
Future Directions

• Stabilize pressure during pulse
• Design / install radiation shielding
• Design / install sample interlock
• Maintain high vacuum (10-8T)
• No need to recondition cathode
• Install sample holder / target
• Adequately cool sample
• Increase radiation levels
• Start processing samples

14
Engineered Tungsten Armor Development

• Vacuum Plasma Spray (VPS) forming techniques are
  being used to produce engineered tungsten armor.
• The engineered tungsten is comprised of a primary
  tungsten undercoat and a nanoporous tungsten
  topcoat.
• Nanometer tungsten feedstock powder is being used
  to produce the nanoporous tungsten topcoat.
• The resulting nanoporous topcoat allows helium
  migration to the
• surface preventing premature failure.

Nanoporous W Topcoat
Primary W Layer
Low Activation Ferritic Steel
Schematic showing the VPSing of the engineered W
armor.
SEM image showing nanometer W feedstock powder
produced by thermal plasma processing. Analysis
has shown the average particle size is less than
100nm. This is one of two nanometer W feedstock
materials used to produce the nanoporous topcoat.
15
Retention of Monoenergetic Helium

• Implanted 1019 3He/m2 (1.3 MeV) at 850C
  followed by a flash anneal at 2000C
• Same total dose was implanted in 1, 10, 100, and
  1000 cycles of implantation and flash heating

• 2.5 MV Van de Graaff acceleratorImplant 3He
  Threat Spectrum by Degrading Energy of Mono
  energetic beam at various temperature and flash
  heating at 20000 C.
• 20 MW Nuclear Reactor- Cold neutron
  sourceMeasure helium retention by neutron depth
  profiling (NDP) technique (NIST, Gaithersburg, MD)

1 i/a
10 i/a
100 i/a
1000 i/a
Relative 3He retention for single crystal and
polycrystalline tungsten with a total dose of
1019 He/m2. Percentage of retained 3He compared
to implanting and annealing in a single cycle.
16
He retention comparisons for 1e1020
3He/m2Nano-Cavity W(lt100nm Particles)
17
Comparison of He Retention with W-
micro-Structurefor Single and multi-Step
Implants and Anneal
Flash Heated to 20000 C for 5 s between the
Steps
18
Results of Surface Blistering/Exfoliation Study
Comparison between Poly-W vs. nano-cavity W.

• Samples Implanted at 8500 C at High Dose of He
  and then Heated to 20000 C for 10 s.
• Single implant/anneal

19
Exfoliation- Poly-W vs. Nano-Cavity W
Poly-W with 2 x 1021 He/m2 in 1 step
Ploly-W with 1 x 1022 He/m2 in 1 step

nano-W unimplanted
nano-W with 5e21 He/m2 in 1 step
(gt100 hrs)
20
5e21 He/m2 in Nano-Cavity W
21
Surface Exfoliation Resultsof Poly-W vs.
Nano-porous W

• Poly-W with 2 x 1021 He/m2 showed blistering
• Poly-W with 1 x 1022 He/m2 showed exfoliation
• Nano-cavity W with 5x 1021 He/m2 did not show
  Surface blistering or exfoliation

Iwakiri 2000
5000 nm
22
Nanoporous W He Retention Remarks

• Helium retention in tungsten is a strong function
  of the amount of helium implanted prior to
  annealing. For HAPL-relevant implant/anneal
  conditions polycrystalline materials exhibit more
  retention than single crystal. This indicates
  that microsctructure and perhaps impurities may
  be important factor in helium retention.
• Nanoporous tungsten appears to have significantly
  less retention than both polycrystalline and
  single crystal tungsten. This result is
  supported by the lack of blistering / exfoliation
  in the as-implanted surface, supporting reduced
  path length argument for mitigating helium
  retention. These results should be considered
  preliminary.
• Further study into the release kinetics, as well
  as the structure of the implanted surface is
  required, though results are encouraging for this
  material system.
• --gt need for thermal desorption analysis
• --gt need to reproduce cyclic implantation/anneal
  study for nanoporous materials
• --gt need to move to more relevant annealing
  condition (degraded helium deposition and laser
  annealing with in-situ thermal desorption.)